ABSTRACT This study investigates experimentally and theoretically the existence slugs for vertical co-current two-phase flow in large diameter vertical tubing and risers. The slug flow pattern implies the presence of Taylor bubbles separated by hydraulically sealed liquid slugs. Previous experimental studies such as Ombere-Ayari and Azzopardi (2007)  showed evidence of the non-existence of Taylor bubbles for large diameter pipes. Models developed to predict this experimental behaviour (Kocamustafaogullari et al., 1984 ; Jayanti and Hewitt , 1990 and Kjølaas et al ., 2017 ) suggest that Taylor bubbles may disappear at large diameters and/or high velocities. However, the previously proposed models do not properly account for the simultaneous effect of velocity and diameter on the stability of a Taylor bubble.